Marathon runners were warned that extreme exercise can damage the heart with
'scars' found in almost one in ten of finishers, research has found.

Experts said exercising for between 30 and 60 minutes a day is ideal and beyond that would lead to 'diminishing returns'.

People who run marathons and cycle long distances risk long-term damage to their hearts and are at greater risk of suffering a heart attack in the two years after their race, they were warned.

A review of research on endurance exercise conducted by a team at the respected Mayo Clinic in Rochester, America, found extreme endurance exercise such as marathons, iron man distance triathlons, and very long distance bicycle races may cause structural changes to the heart and large arteries.

The Daily Telegraph revealed last week that surgeons are seeing an increase in the number of middle-aged fitness fans who are wearing out their knee joints by playing tennis and running into their 40s and 50s.

Published in the journal Mayo Clinic Proceedings it was found that some athletes suffer temporary changes in their heart function which return to normal in the week after their race, however for others permanent scarring occurs.

Lead author Dr James H. O'Keefe, of Saint Luke's Hospital of Kansas City, said: "Physically active people are much healthier than their sedentary counterparts.

"Exercise is one of the most important things you need to do on a daily basis.

"But what this paper points out is that a lot of people do not understand that the lion's share of health benefits accrue at a relatively modest level. Extreme exercise is not really conducive to great cardiovascular health. Beyond 30-60 minutes per day, you reach a point of diminishing returns."

He added: "Physical exercise, though not a drug, possesses many traits of a powerful pharmacologic agent.

"A routine of daily physical activity can be highly effective for prevention and treatment of many diseases, including coronary heart disease, hypertension, heart failure, and obesity.

"However, as with any pharmacologic agent, a safe upper dose limit potentially exists, beyond which the adverse effects of physical exercise, such as musculoskeletal trauma and cardiovascular stress, may outweigh its benefits."

As well as scarring of the heart muscle, elite athletes can develop changes in their heart rhythm which can predispose them to sudden cardiac arrest and death if not treated quickly.

Endurance sports have been linked to a five-fold increased risk of atrial fibrillation, a heart rhythm disturbance which requires careful treatment and can be fatal.

More research is needed to establish the level at which exercise becomes harmful to the heart so exercise programmes can be devised to maximise the health benefits while protecting the heart, Dr O'Keefe said.